1. Field of the Invention
The present invention relates to active materials for the positive electrode in an alkaline storage battery which are mainly composed of a metal oxide containing Ni as a main metallic element, and which are high in capacity, and a manufacturing method therefor.
2. Description of the Related Art
With the recent progress of semiconductor techniques, miniaturization, weight-saving and multi-functionation of electronic apparatuses have advanced and personalization of small-sized portable equipment represented by portable telephone, notebook type personal computer or the like is rapidly progressed. Therefore, demand for miniaturization and weight-saving of alkaline storage batteries which are widely employed as electric sources of them is increased.
Up to now, nickel oxide (NiOOH) has been used as the main active material for positive electrode in alkaline storage batteries while in place of sintered type electrodes using conventional sintered substrate there has been industrialized foamed metal type electrodes comprising a three-dimensional foamed nickel porous body of higher porosity (about 95%) and nickel oxide powders filled in the nickel porous body at high density (see, for example, JP-B-62-54235 and U.S. Pat. No. 4,251,603), whereby energy density of nickel positive electrodes has been markedly increased.
For the increase in energy density of nickel positive electrodes, improvement of manufacturing method of nickel oxide powder which is an active material is also one of important techniques. Conventional manufacturing method of nickel oxide powder comprises reacting an aqueous alkaline solution such as sodium hydroxide with an aqueous solution of a nickel salt to produce a precipitate, then aging it to grow crystal, and thereafter grinding the crystal by a mechanical method. However, this method is troublesome and besides since the resulting powders are irregular shape, a high packing density can hardly be obtained. However, as shown in JP-B-4-80513, another method has been proposed which comprises reacting an aqueous solution of a nickel salt with ammonia to form an ammonium complex of nickel and growing nickel hydroxide in an aqueous alkaline solution. According to this method, continuous production becomes possible, resulting in reduction of cost and furthermore since the resulting powders are close to spherical in shape high density packing becomes possible.
However, since high density particles of large size growing to several ten .mu.m are used as the active material, charge and discharge efficiency deteriorates owing to the decrease of electronic conductivity of the active material per se. For the solution of this problem, the electronic conductivity is supplemented by adding Co or oxides thereof and Ni (JP-B-61-37733, "Denki Kagaku", Vol. 54, No. 2, p. 159 (1986), "Power Sources", 12, p 203 (1988)), and, furthermore, improvement of charge and discharge efficiency is attempted by dissolving metallic elements such as Co other than Ni in the active material to form a solid solution.
Moreover, as the attempt to improve charge and discharge efficiency by dissolving different metallic elements in crystals as mentioned above, a method of adding Cd or Co in the active material to form a solid solution (e.g., JP-B-3-26903, JP-B-3-50384, "Denki Kagaku", Vol. 54, No. 2, p. 164 (1986), "Power Sources", 12, p 203 (1988)) is employed, but from the environmental viewpoint, batteries free of cadmium are desired, and Zn is proposed as one example of metallic elements substituted for cadmium and, moreover, a solid solution of three elements such as Co, Zn and Ba is proposed (U.S. Pat. No. 5,366,831). The dissolution of different metals to a nickel oxide in the form of solid solution for the purpose of enhancement of efficiency of charge-discharge efficiency is an old technique and is known in JP-A-51-122737.
Energy density of positive electrode has been markedly increased by the above-mentioned improvement in structure of substrates, shape of active material particles, composition of active material and additives thereto, and, at present, positive electrodes of about 600 mAh/cc in energy density are put to practical use. However, improvement of energy density as electric source for small-sized portable equipment is being increasingly demanded. In order to attain improvement of energy density of batteries, an approach is considered on positive and negative electrodes, electrolyte, separator or construction of them. As for negative electrode, a volume energy density more than twice that of positive electrode has been obtained by the practical use of metal hydrides of high energy density in place of conventional cadmium negative electrodes ("Power Sources", 12, p. 393(1988)). With respect to construction of batteries, rapid increase of energy density has been made by the technical progresses such as reduction in thickness of separator and packing of active materials in electrode plates at high density, and, at present, the increase of energy density reaches nearly a limit. For the realization of further improvement of energy density, it is important as the most effective technique to further increase energy density of positive electrode which occupies nearly a half of volume in a battery.
In order to improve energy density of positive electrode, there can be considered approaches to improve packing density of electrode such as improvement of tap density of active material, reduction of the amount of additives, and reduction of amount of metal in the foamed nickel substrate, but these techniques reach nearly a limit. Therefore, it is necessary to attempt improvement in reactivity and order of reaction by improving the active material per se. It is said that a nickel oxide which is an active material for positive electrode at present is a .beta.-type Ni(OH).sub.2 (divalent oxide) at packing and one-electron reaction (utilization factor: 100%) with a .beta.-type NiOOH (trivalent) proceeds at usual charge and discharge. However, a part of this .beta.-NiOOH in charged state is oxidized to .gamma.-NiOOH which is an oxide of higher valence (3.5-3.8 valences) by overcharge. It is known that at least .gamma.-NiOOH is a non-stoichiometric material and is crystallographically disorder ("J. Power Sources", 8, p 229 (1982)). Hitherto, an attempt has been made to inhibit production of .gamma.-NiOOH because this is electrochemically inactive and causes various problems such as voltage drop or capacity decrease and, in addition, insufficient contact with conductive agent, substrate and others due to volume expansion of electrode resulting from interlaminar extension, falling off of active material, and exhaustion of electrolyte due to inclusion of water molecule.
However, for the further increase of energy density using an active material comprising a nickel oxide as a base, it is very important to properly utilize .gamma.-NiOOH which is an oxide of higher valence. For this purpose, materials have been proposed which have a structure close to .alpha. type hydroxide including anion and water molecule between layers and which are prepared by dissolving different metals such as Mn (III), Al (III), and Fe (III) in the state of solid solution in place of a part of Ni ("Solid State Ionics", 32/33, p. 104 (1989), "J. Power Sources", 35, p. 249(1991), U.S. Pat. Nos. 5,348,822(1994) and 5,569,562(1996), JP-A-8-225328)). It is said that charge and discharge reactions easily proceed between these oxides and the oxides of higher valence having a structure similar to that of .gamma.-NiOOH. However, it is considered that actually these oxides have wide interlaminar space resulting in very high bulkiness, and, hence, high density packing is difficult and they are poor in utility.
On the other hand, the inventors have paid attention to active materials which have a .beta.-type crystal structure at the time of packing in electrode and undergo charge and discharge reaction with .gamma.-NiOOH which is an oxide of higher valence. As one example, we have proposed modification of nickel oxides by the dissolution of a different metal in solid state for attaining high density and higher reaction. Furthermore, we have proposed that a composition mainly composed of Mn is especially hopeful as the different metal to be dissolved (e.g., JP-A-8-222215, JP-A-8-222216 and JP-A-9-115543). These disclose that mobility of proton and electronic conductivity are improved and utilization factor is increased by dissolving Mn in a nickel oxide in the state of solid solution. Nickel oxides in which Mn is dissolved in the state of solid solution are proposed in JP-A-51-122737, JP-A-4-179056 and JP-A-41212.
As mentioned above, some attempts have been proposed to modify nickel oxides by dissolving a different metal in solid solution, thereby to improve charge and discharge efficiency. However, in some cases, the effect cannot be sufficiently exhibited depending on the kind of metals to be dissolved and amount of the metals dissolved. On the other hand, in the case of nickel oxides in which Mn is dissolved in solid solution, the effects of improvement in charge and discharge efficiency and order of reaction are considerably great, and improvement in energy density can be expected. However, JP-A-4-179056 and JP-A-5-41212 mainly aim at improvement of cycle life and do not aim at improvement of order of reaction.
Moreover, JP-A-8-222215, JP-A-8-222216 and JP-A-9-115543 do not disclose proper values of valence of Mn, crystal structure and pore distribution which have a great influence on battery characteristics, and further improvement must be performed for attaining the higher energy density. In addition, in preparation of the nickel oxide in which Mn is dissolved to form a solid solution, Mn (II) is unstable and readily oxidized, and it is very difficult to grow the oxide to high density particles. However, none of the above proposals disclose solution of this problem, and realization of high energy density is difficult. Moreover, U.S. Pat. No. 5,637,423 proposes Ni(OH).sub.2 containing Mn, but discloses production of only sintered electrode plates and does not disclose production of powdered metal oxide for attaining high energy density.